Build a scalable front-end with Rush monorepo and React — Webpack + Jest

TL;DR

If you're interested in just see the code, you can find it here: https://github.com/abereghici/rush-monorepo-boilerplate

If you want to see an example with Rush used in a real, large project, you can look at ITwin.js, an open-source project developed by Bentley Systems.

In this post, we'll create our own build tools, based on create-react-app.
This will give us full control over the build process.

Create react-scripts package

Let's create a new folder for react-scripts package.

mkdir -p packages/react-scripts

Create a package.json file inside the react-scripts folder and add the following configuration:

{
  "name": "@monorepo/react-scripts",
  "version": "1.0.0",
  "files": ["bin", "config", "lib", "scripts", "utils"],
  "bin": {
    "react-scripts": "./bin/react-scripts.js"
  },
  "types": "./lib/react-app.d.ts",
  "dependencies": {
    "@babel/core": "7.12.3",
    "@pmmmwh/react-refresh-webpack-plugin": "0.4.3",
    "@svgr/webpack": "5.5.0",
    "@testing-library/jest-dom": "^5.14.1",
    "@testing-library/react": "^11.2.7",
    "@testing-library/user-event": "^12.8.3",
    "@types/jest": "^26.0.24",
    "@types/node": "^12.20.19",
    "@types/react": "^17.0.18",
    "@types/react-dom": "^17.0.9",
    "@typescript-eslint/eslint-plugin": "^4.5.0",
    "@typescript-eslint/parser": "^4.5.0",
    "babel-eslint": "^10.1.0",
    "babel-jest": "^26.6.0",
    "babel-loader": "8.1.0",
    "babel-plugin-named-asset-import": "^0.3.7",
    "babel-preset-react-app": "^10.0.0",
    "bfj": "^7.0.2",
    "camelcase": "^6.1.0",
    "case-sensitive-paths-webpack-plugin": "2.3.0",
    "css-loader": "4.3.0",
    "dotenv": "8.2.0",
    "dotenv-expand": "5.1.0",
    "file-loader": "6.1.1",
    "fs-extra": "^9.0.1",
    "html-webpack-plugin": "4.5.0",
    "identity-obj-proxy": "3.0.0",
    "jest": "26.6.0",
    "jest-circus": "26.6.0",
    "jest-resolve": "26.6.0",
    "jest-watch-typeahead": "0.6.1",
    "mini-css-extract-plugin": "0.11.3",
    "optimize-css-assets-webpack-plugin": "5.0.4",
    "pnp-webpack-plugin": "1.6.4",
    "postcss-flexbugs-fixes": "4.2.1",
    "postcss-loader": "3.0.0",
    "postcss-normalize": "8.0.1",
    "postcss-preset-env": "6.7.0",
    "postcss-safe-parser": "5.0.2",
    "prompts": "2.4.0",
    "react-app-polyfill": "^2.0.0",
    "react-dev-utils": "^11.0.3",
    "react-refresh": "^0.8.3",
    "resolve": "1.18.1",
    "resolve-url-loader": "^3.1.2",
    "sass-loader": "^10.0.5",
    "semver": "7.3.2",
    "style-loader": "1.3.0",
    "terser-webpack-plugin": "4.2.3",
    "ts-pnp": "1.2.0",
    "url-loader": "4.1.1",
    "web-vitals": "^1.1.2",
    "webpack": "4.44.2",
    "webpack-dev-server": "3.11.1",
    "webpack-manifest-plugin": "2.2.0",
    "workbox-webpack-plugin": "5.1.4"
  },
  "devDependencies": {
    "react": "^17.0.1",
    "react-dom": "^17.0.1"
  }
}

As you already noticed, we moved all build dependencies from react-app project to react-scripts package.

Move config and scripts folders from react-app to react-scripts and let's adjust the configurations.

Remove ESLint plugin and its dependencies from packages/react-scripts/config/webpack.config.js. We'll use it later as a separate action in our build process.

Open packages/react-scripts/config/paths.js and replace the file content with:

const path = require('path');
const fs = require('fs');
const getPublicUrlOrPath = require('react-dev-utils/getPublicUrlOrPath');

// Make sure any symlinks in the project folder are resolved:
// https://github.com/facebook/create-react-app/issues/637
const appDirectory = fs.realpathSync(process.cwd());
const resolveApp = relativePath => path.resolve(appDirectory, relativePath);

// We use `PUBLIC_URL` environment variable or "homepage" field to infer
// "public path" at which the app is served.
// webpack needs to know it to put the right <script> hrefs into HTML even in
// single-page apps that may serve index.html for nested URLs like /todos/42.
// We can't use a relative path in HTML because we don't want to load something
// like /todos/42/static/js/bundle.7289d.js. We have to know the root.
const publicUrlOrPath = getPublicUrlOrPath(
  process.env.NODE_ENV === 'development',
  require(resolveApp('package.json')).homepage,
  process.env.PUBLIC_URL
);

const buildPath = process.env.BUILD_PATH || 'build';

const moduleFileExtensions = [
  'web.mjs',
  'mjs',
  'web.js',
  'js',
  'web.ts',
  'ts',
  'web.tsx',
  'tsx',
  'json',
  'web.jsx',
  'jsx',
];

// Resolve file paths in the same order as webpack
const resolveModule = (resolveFn, filePath) => {
  const extension = moduleFileExtensions.find(extension =>
    fs.existsSync(resolveFn(`${filePath}.${extension}`))
  );

  if (extension) {
    return resolveFn(`${filePath}.${extension}`);
  }

  return resolveFn(`${filePath}.js`);
};

const resolveOwn = relativePath => path.resolve(__dirname, '..', relativePath);

module.exports = {
  dotenv: resolveApp('.env'),
  appPath: resolveApp('.'),
  appBuild: resolveApp(buildPath),
  appPublic: resolveApp('public'),
  appHtml: resolveApp('public/index.html'),
  appIndexJs: resolveModule(resolveApp, 'src/index'),
  appPackageJson: resolveApp('package.json'),
  appSrc: resolveApp('src'),
  appTsConfig: resolveApp('tsconfig.json'),
  appJsConfig: resolveApp('jsconfig.json'),
  yarnLockFile: resolveApp('yarn.lock'),
  testsSetup: resolveModule(resolveApp, 'src/setupTests'),
  proxySetup: resolveApp('src/setupProxy.js'),
  appNodeModules: resolveApp('node_modules'),
  appWebpackCache: resolveApp('node_modules/.cache'),
  appTsBuildInfoFile: resolveApp('node_modules/.cache/tsconfig.tsbuildinfo'),
  swSrc: resolveModule(resolveApp, 'src/service-worker'),
  publicUrlOrPath,
  // These properties only exist before ejecting:
  ownPath: resolveOwn('.'),
  ownNodeModules: resolveOwn('node_modules'), // This is empty on npm 3
  appTypeDeclarations: resolveApp('src/react-app-env.d.ts'),
  ownTypeDeclarations: resolveOwn('lib/react-app.d.ts'),
};

module.exports.moduleFileExtensions = moduleFileExtensions;

The most important change in this file is the resolveApp function call.
We shouldn't use relative paths anymore, because we are moving the configuration files to react-scripts package.
We'll get the project files paths using a combination between path.resolve and process.cwd().

Create a bin folder and in react-scripts.js file add the following code:

process.on('unhandledRejection', err => {
  throw err;
});

const spawn = require('react-dev-utils/crossSpawn');
const args = process.argv.slice(2);

const scriptIndex = args.findIndex(
  x => x === 'build' || x === 'start' || x === 'test'
);
const script = scriptIndex === -1 ? args[0] : args[scriptIndex];
const nodeArgs = scriptIndex > 0 ? args.slice(0, scriptIndex) : [];

if (['build', 'start', 'test'].includes(script)) {
  const result = spawn.sync(
    process.execPath,
    nodeArgs
      .concat(require.resolve('../scripts/' + script))
      .concat(args.slice(scriptIndex + 1)),
    { stdio: 'inherit' }
  );
  if (result.signal) {
    if (result.signal === 'SIGKILL') {
      console.log(
        'The build failed because the process exited too early. ' +
          'This probably means the system ran out of memory or someone called ' +
          '`kill -9` on the process.'
      );
    } else if (result.signal === 'SIGTERM') {
      console.log(
        'The build failed because the process exited too early. ' +
          'Someone might have called `kill` or `killall`, or the system could ' +
          'be shutting down.'
      );
    }
    process.exit(1);
  }
  process.exit(result.status);
} else {
  console.log('Unknown script "' + script + '".');
}

This will be the entry-point of our CLI. Every time when you'll want to add a new command you have to adjust this file.

Create another folder named lib and in react-app.d.ts file add the following code:

/// <reference types="node" />
/// <reference types="react" />
/// <reference types="react-dom" />

declare namespace NodeJS {
  interface ProcessEnv {
    readonly NODE_ENV: 'development' | 'production' | 'test';
    readonly PUBLIC_URL: string;
  }
}

declare module '*.avif' {
  const src: string;
  export default src;
}

declare module '*.bmp' {
  const src: string;
  export default src;
}

declare module '*.gif' {
  const src: string;
  export default src;
}

declare module '*.jpg' {
  const src: string;
  export default src;
}

declare module '*.jpeg' {
  const src: string;
  export default src;
}

declare module '*.png' {
  const src: string;
  export default src;
}

declare module '*.webp' {
  const src: string;
  export default src;
}

declare module '*.svg' {
  import * as React from 'react';

  export const ReactComponent: React.FunctionComponent<
    React.SVGProps<SVGSVGElement> & { title?: string }
  >;

  const src: string;
  export default src;
}

declare module '*.module.css' {
  const classes: { readonly [key: string]: string };
  export default classes;
}

declare module '*.module.scss' {
  const classes: { readonly [key: string]: string };
  export default classes;
}

declare module '*.module.sass' {
  const classes: { readonly [key: string]: string };
  export default classes;
}

Here we'll store all type definitions for css, sass, scss and image files and we'll reuse in all react applications inside of the monorepo. You can address to these type definitions by creating a file in your react project named react-app-env.d.ts with the following content:

/// <reference types="@monorepo/react-scripts" />

Now let's register @monorepo/react-scripts package in rush configuration file. Open rush.json file and add an entry like this under the projects inventory:

. . .
 "projects": [
    {
      "packageName": "@monorepo/react-scripts",
      "projectFolder": "packages/react-scripts",
      "reviewCategory": "tools"
    }
  ]
. . .

Now, let's cleanup apps/react-app/package-json file by removing all dependencies related to build tools. We'll also remove the configurations for eslint, babel or jest, because we'll store them in a separate files. In our dependencies we'll list @monorepo/react-scripts package.

Edit apps/react-app/package.json file and add the following configuration:

{
  "name": "@monorepo/react-app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^5.14.1",
    "@testing-library/react": "^11.2.7",
    "@testing-library/user-event": "^12.8.3",
    "@types/jest": "^26.0.24",
    "@types/react": "^17.0.18",
    "@types/react-dom": "^17.0.9",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "typescript": "^4.3.5",
    "web-vitals": "^1.1.2",
    "@monorepo/react-scripts": "1.0.0"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test"
  }
}

Run rush update to install all dependencies and let's try to build our app with the react-script package we just created.

cd apps/react-app

rushx build

Setup BrowserList

By default, react-scripts expect to find a browserslist configuration in each project, otherwise, it fallbacks to the default browserslist configuration. If you want to use the same configuration for all your projects, you can modify packages/react-scripts/build.js to use browserslist from package.json from @monorepo/react-scripts.

Open package.json from @monorepo/react-scripts and add the following configuration:

"browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  },

Modify the checkBrowsers function call from packages/react-scripts/build.js and packages/react-scripts/start.js with:

checkBrowsers(paths.ownPath, isInteractive)

Setup Jest

Our current test script expects to find a jest configuration in each project. We'll modify it to use a shared configuration.

Create a utils folder in packages/react-scripts/scripts and in createJestConfig.js file add the following:

const fs = require('fs');
const chalk = require('react-dev-utils/chalk');
const paths = require('../../config/paths');
const modules = require('../../config/modules');

module.exports = (resolve, rootDir, isEjecting) => {
  // Use this instead of `paths.testsSetup` to avoid putting
  // an absolute filename into configuration after ejecting.
  const setupTestsMatches = paths.testsSetup.match(/src[/\\]setupTests\.(.+)/);
  const setupTestsFileExtension =
    (setupTestsMatches && setupTestsMatches[1]) || 'js';
  const setupTestsFile = fs.existsSync(paths.testsSetup)
    ? `<rootDir>/src/setupTests.${setupTestsFileExtension}`
    : undefined;

  const config = {
    roots: ['<rootDir>/src'],

    collectCoverageFrom: ['src/**/*.{js,jsx,ts,tsx}', '!src/**/*.d.ts'],

    setupFiles: [
      isEjecting
        ? 'react-app-polyfill/jsdom'
        : require.resolve('react-app-polyfill/jsdom'),
    ],

    setupFilesAfterEnv: setupTestsFile ? [setupTestsFile] : [],
    testMatch: [
      '<rootDir>/src/**/__tests__/**/*.{js,jsx,ts,tsx}',
      '<rootDir>/src/**/*.{spec,test}.{js,jsx,ts,tsx}',
    ],
    testEnvironment: 'jsdom',
    testRunner: require.resolve('jest-circus/runner'),
    transform: {
      '^.+\\.(js|jsx|mjs|cjs|ts|tsx)$': resolve(
        'config/jest/babelTransform.js'
      ),
      '^.+\\.css$': resolve('config/jest/cssTransform.js'),
      '^(?!.*\\.(js|jsx|mjs|cjs|ts|tsx|css|json)$)': resolve(
        'config/jest/fileTransform.js'
      ),
    },
    transformIgnorePatterns: [
      '[/\\\\]node_modules[/\\\\].+\\.(js|jsx|mjs|cjs|ts|tsx)$',
      '^.+\\.module\\.(css|sass|scss)$',
    ],
    modulePaths: modules.additionalModulePaths || [],
    moduleNameMapper: {
      '^react-native$': 'react-native-web',
      '^.+\\.module\\.(css|sass|scss)$': 'identity-obj-proxy',
      ...(modules.jestAliases || {}),
    },
    moduleFileExtensions: [...paths.moduleFileExtensions, 'node'].filter(
      ext => !ext.includes('mjs')
    ),
    watchPlugins: [
      'jest-watch-typeahead/filename',
      'jest-watch-typeahead/testname',
    ],
    resetMocks: true,
  };
  if (rootDir) {
    config.rootDir = rootDir;
  }
  const overrides = Object.assign({}, require(paths.appPackageJson).jest);
  const supportedKeys = [
    'clearMocks',
    'collectCoverageFrom',
    'coveragePathIgnorePatterns',
    'coverageReporters',
    'coverageThreshold',
    'displayName',
    'extraGlobals',
    'globalSetup',
    'globalTeardown',
    'moduleNameMapper',
    'resetMocks',
    'resetModules',
    'restoreMocks',
    'snapshotSerializers',
    'testMatch',
    'transform',
    'transformIgnorePatterns',
    'watchPathIgnorePatterns',
  ];
  if (overrides) {
    supportedKeys.forEach(key => {
      if (Object.prototype.hasOwnProperty.call(overrides, key)) {
        if (Array.isArray(config[key]) || typeof config[key] !== 'object') {
          // for arrays or primitive types, directly override the config key
          config[key] = overrides[key];
        } else {
          // for object types, extend gracefully
          config[key] = Object.assign({}, config[key], overrides[key]);
        }

        delete overrides[key];
      }
    });
    const unsupportedKeys = Object.keys(overrides);
    if (unsupportedKeys.length) {
      const isOverridingSetupFile =
        unsupportedKeys.indexOf('setupFilesAfterEnv') > -1;

      if (isOverridingSetupFile) {
        console.error(
          chalk.red(
            'We detected ' +
              chalk.bold('setupFilesAfterEnv') +
              ' in your package.json.\n\n' +
              'Remove it from Jest configuration, and put the initialization code in ' +
              chalk.bold('src/setupTests.js') +
              '.\nThis file will be loaded automatically.\n'
          )
        );
      } else {
        console.error(
          chalk.red(
            '\nOut of the box, Create React App only supports overriding ' +
              'these Jest options:\n\n' +
              supportedKeys
                .map(key => chalk.bold('  \u2022 ' + key))
                .join('\n') +
              '.\n\n' +
              'These options in your package.json Jest configuration ' +
              'are not currently supported by Create React App:\n\n' +
              unsupportedKeys
                .map(key => chalk.bold('  \u2022 ' + key))
                .join('\n') +
              '\n\nIf you wish to override other Jest options, you need to ' +
              'eject from the default setup. You can do so by running ' +
              chalk.bold('npm run eject') +
              ' but remember that this is a one-way operation. ' +
              'You may also file an issue with Create React App to discuss ' +
              'supporting more options out of the box.\n'
          )
        );
      }

      process.exit(1);
    }
  }
  return config;
};

Now let's use this configuration. Modify the test script in packages/react-scripts/scripts/test.js to look like this:

// Do this as the first thing so that any code reading it knows the right env.
process.env.BABEL_ENV = 'test';
process.env.NODE_ENV = 'test';
process.env.PUBLIC_URL = '';

process.on('unhandledRejection', err => {
  throw err;
});

// Ensure environment variables are read.
require('../config/env');

const jest = require('jest');
const execSync = require('child_process').execSync;
let argv = process.argv.slice(2);

function isInGitRepository() {
  try {
    execSync('git rev-parse --is-inside-work-tree', { stdio: 'ignore' });
    return true;
  } catch (e) {
    return false;
  }
}

function isInMercurialRepository() {
  try {
    execSync('hg --cwd . root', { stdio: 'ignore' });
    return true;
  } catch (e) {
    return false;
  }
}

// Watch unless on CI or explicitly running all tests
if (
  !process.env.CI &&
  argv.indexOf('--watchAll') === -1 &&
  argv.indexOf('--watchAll=false') === -1
) {
  // https://github.com/facebook/create-react-app/issues/5210
  const hasSourceControl = isInGitRepository() || isInMercurialRepository();
  argv.push(hasSourceControl ? '--watch' : '--watchAll');
}

const createJestConfig = require('./utils/createJestConfig');
const path = require('path');
const paths = require('../config/paths');
argv.push(
  '--config',
  JSON.stringify(
    createJestConfig(
      relativePath => path.resolve(__dirname, '..', relativePath),
      path.resolve(paths.appSrc, '..'),
      false
    )
  )
);

// This is a very dirty workaround for https://github.com/facebook/jest/issues/5913.
// We're trying to resolve the environment ourselves because Jest does it incorrectly.
// TODO: remove this as soon as it's fixed in Jest.
const resolve = require('resolve');
function resolveJestDefaultEnvironment(name) {
  const jestDir = path.dirname(
    resolve.sync('jest', {
      basedir: __dirname,
    })
  );
  const jestCLIDir = path.dirname(
    resolve.sync('jest-cli', {
      basedir: jestDir,
    })
  );
  const jestConfigDir = path.dirname(
    resolve.sync('jest-config', {
      basedir: jestCLIDir,
    })
  );
  return resolve.sync(name, {
    basedir: jestConfigDir,
  });
}
let cleanArgv = [];
let env = 'jsdom';
let next;
do {
  next = argv.shift();
  if (next === '--env') {
    env = argv.shift();
  } else if (next.indexOf('--env=') === 0) {
    env = next.substring('--env='.length);
  } else {
    cleanArgv.push(next);
  }
} while (argv.length > 0);
argv = cleanArgv;
let resolvedEnv;
try {
  resolvedEnv = resolveJestDefaultEnvironment(`jest-environment-${env}`);
} catch (e) {
  // ignore
}
if (!resolvedEnv) {
  try {
    resolvedEnv = resolveJestDefaultEnvironment(env);
  } catch (e) {
    // ignore
  }
}
const testEnvironment = resolvedEnv || env;
argv.push('--env', testEnvironment);

jest.run(argv);